The overall objectives of the project are: (1) To purify the enzymes responsible for host-controlled modification and restriction of DNA from E. coli B verify their specificity in vitro. (2) To determine the subunit structure of the enzymes and to try to understand how these subunits interact to form enzymes which act at specific sites on the DNA. (3) To study the enzyme mechanisms and the role of the enzyme cofactors. (4) To determine the DNA sequence which the enzyme recognizes. (5) To study the activities of several newly-detected endonucleases which are byproducts of the restriction and modification enzyme preparations, and to determine the function of these enzymes. (6) To study how these enzymes can be used as reagents for studying DNA structure.